1. Field of the Invention
The present invention relates to systems and methods for detecting gaseous leaks. More specifically, the present invention relates to systems and methods for detecting gas leaks using a mass spectrometer.
2. Description of the Related Art
Some missile systems fully enclose the projectile in a hermetically sealed storage tube in order to physically protect the missile and limit permeation of water into the container. In some systems, the storage tube also serves as a launch tube for the missile. If water should migrate into the missile, it could seriously damage the system (for instance, through corrosion, condensation on the electronics or optics, etc.). During the manufacturing process, the storage/launch tube is therefore hermetically sealed and must be leak checked to ensure that it is sealed to the level required by the system.
The most common method of leak detection involves injecting a tracer gas such as helium around the unit under test and using a mass spectrometer to detect if any of the tracer gas has leaked into the unit. This type of leak detection is very accurate, sensitive, and can quantitatively measure the leak rate of the unit. A mass spectrometer, however, typically requires creating a vacuum in the unit.
Certain missile storage/launch tubes cannot be subject to the 1 atm differential pressure encountered with vacuum leak detection equipment. This is because the storage/launch tube includes end caps that are designed to burst when exposed to pressure or are weakly attached to the tube body (so that the missile can exit the tube during launch). Creating a vacuum inside the tube (or in a test chamber containing the tube) in order to perform a mass spectrometer leak detection test would cause the end caps to burst. This limitation on the differential pressure that can be applied to the tube limits the kinds of leak detection that can be used.
Currently, missile storage/launch tubes are typically leak checked using a pressure decay method. This method involves increasing the pressure inside the tube by a very small amount (typically, about 5 psi) and then measuring the pressure after a certain amount of time has passed (typically, about 45 minutes). If the pressure has decayed by more than a specified amount, then the tube is considered to have an unacceptably high leak rate.
The pressure decay test, however, is relatively insensitive, inaccurate, and slow when compared to mass spectrometer leak detection. Since the test depends on the measured change in pressure, it is very sensitive to temperature changes. Even a fraction of a degree of change in the ambient temperature can cause a false indication of a leak, or worse, mask a real leak. In order to help control the temperature, the missile storage/launch tube is typically placed in a test chamber and allowed to sit for 2 hours in order to come to thermal equilibrium. The pressure decay test is then applied, which takes another 45 minutes. The entire procedure therefore takes about 2 hours and 45 minutes, an excessively long amount of time that can be very expensive (due to labor and equipment costs). The pressure decay test is also very imprecise, since the pressure change being measured is a very small amount. The tube cannot be pumped to higher pressure levels to increase precision because the tubes cannot be subject to larger amounts of pressure, as discussed above.
Hence, a need exists in the art for an improved system or method for detecting leaks in systems that are sensitive to differential pressure that is more accurate and less time consuming than prior approaches.